I know I’m late to the party but I just found out and didn’t find a post regarding this
TL;DR: Steam now calculates review scores based on reviews made on your languages only (if enough). This is on by default, but can be reverted to go back to review scores based on all reviews (from people that purchased the game directly on Steam only)
It’s worth adding that you can disable this filter permanently. It’s not a thing you temporarily disable while you’re on a specific game’s store page; if you disable it, it disables it for every game until you turn it back on. So that’s nice.